The present invention relates generally to radiotelephones, and, more particularly, to retractable antenna systems for use with portable radiotelephones.
Radiotelephones, which are well known in the art, generally refer to communications terminals which can provide a wireless communications link (including optionally both voice and data) to one or more other communications terminals. Such radiotelephones are used in a variety of different applications, including cellular telephone, land-mobile (e.g., police and fire departments), and satellite communications systems.
Cellular telephone systems are commonly employed to provide voice and data communications to a plurality of subscribers within a prescribed geographic area. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. Recently, digital cellular radiotelephone systems such as those designated IS-54B (and its successor IS-136) in North America and GSM in Europe have been introduced and are currently being deployed. These systems, and others, are described, for example, in the book entitled Cellular Radio Systems, by Balston, et al., published by Artech House, Norwood, Mass. (1993). In addition to the above systems, an evolving system referred to as Personal Communication Services (PCS) is being implemented. Examples of current PCS systems include those designated IS-95, PCS-1900, and PACS in North America, DCS-1800 and DECT in Europe, and PHS in Japan. These PCS systems operate at approximately the 2 gigahertz (GHz) band of the radio spectrum, and are typically being used for voice and high bit-rate data communications.
Many radiotelephones, and in particular handheld radiotelephones such as those typically used with cellular telephone systems, employ retractable antennas which may be extended out of, and retracted back into, the radiotelephone housing. Typically, such retractable antennas are electrically connected to a printed circuit board located within the housing of the radiotelephone that contains signal processing and other radio frequency circuitry. In order to maximize the transfer of power between the antenna and this radio frequency circuitry, the antenna and the radio frequency circuitry are typically interconnected such that the impedance of the antenna and the signal processing circuit are substantially matched. As many radiotelephones use 50 ohm impedance coaxial cable or microstrip transmission lines to connect the antenna to the radio frequency circuit, such matching typically comprises mechanically adjusting, electrically tuning or otherwise configuring the antenna so that it exhibits an impedance of approximately 50 ohms at its connection with the coaxial cable or microstrip transmission line.
Unfortunately, however, matching the impedance of a retractable antenna is more difficult, as the impedance exhibited by the antenna is generally dependent on the position of the antenna with respect to both the housing of the radiotelephone and the printed circuit board which contains the radio frequency circuitry. As these respective positions change when the antenna is moved between the extended and retracted positions, the antenna typically exhibits at least two different impedance states, both of which should be matched to the 50 ohm impedance of the feed from the printed circuit board. Accordingly, with retractable antennas, it is generally necessary to provide an impedance matching system that provides an acceptable impedance match between the antenna and the radio frequency circuitry both when the antenna is retracted and extended.
A number of different matching techniques are conventionally used with retractable antennas. For instance, many radiotelephones with retractable antennas employ dual impedance matching circuits, one of which is associated with the extended antenna position and the other with the retracted position. These matching systems typically comprise two or more resonant circuits and switches for switching between these circuits as a function of the position of the antenna. Other radiotelephones only provide a single matching circuit (which is switched in when the antenna is in the extended position), and operate without the benefit of any matching circuit when the antenna is in the retracted position. In other designs, a half-wavelength (xcex/2) antenna may be used so that the antenna radiates as a half-wavelength structure in the extended position and as a quarter-wavelength (xcex/4) antenna in the retracted position (as the retracted portion of the antenna does not radiate). With this arrangement, impedance matching is typically only required in the extended position, as the antenna may be designed to have a natural impedance reasonably close to 50 ohms in the retracted position. Still other radiotelephones use parasitic elements or printed transformer segments to match the impedance of the antenna to the radio frequency circuit board. However, each of the aforementioned techniques typically require some sort of matching means, which in turn requires space within the housing (or antenna) for matching components, and which additionally increases the overall cost of manufacturing the radiotelephone.
The aforementioned matching problems are further compounded in xe2x80x9cdual-bandxe2x80x9d radiotelephones that are designed to transmit and receive signals in two or more widely separated frequency bands. In such phones, it is desirable to provide a single antenna structure that can operate in both bands. For example, a cellular telephone may operate in a conventional analog (AMPS) band at around 800 MHz and also in a PCS band at around 1900 MHz.
As the impedance seen at the base of the antenna is usually a function of frequency, antenna systems for such dual-band radiotelephones may provide separate matching networks for each of the two frequency bands of operation. Accordingly, if retractable antennas are used on such radiotelephones, it is often necessary to provide as many as four matching networks to ensure that an acceptable impedance match is achieved at each frequency of operation and each possible antenna position (i.e., extended or retracted).
Helix antennas provide an advantage over rod or monopole antennas in applications such as cellular telephones as they typically are shorter. This class of antenna refers to antennas which comprise a conducting member wound in a helical pattern. As the conducting member is wound about an axis, the axial length of a quarter-wavelength or half-wavelength helix antenna is considerably less than the length of a comparable quarter-wavelength monopole antenna, and thus helix antennas may often be employed where the length of a quarter-wavelength monopole antenna is prohibitive. Moreover, although a half-wavelength or a quarter-wavelength helix antenna is typically considerably shorter than its half-wavelength or quarter-wavelength monopole antenna counterpart, it may exhibit the same effective electrical length.
Several dual-band helix antenna systems have been proposed. For instance, U.S. Pat. No. 4,554,554 to Olesen et al. discusses a quadrifilar helix antenna which includes PIN diode switches along each of its elements to provide means for selectively resonating the antenna at one of two distinct frequencies by changing the electrical length of the elements.
Similarly, U.S. Pat. No. 4,494,122 to Garay et al. discusses an antenna system comprising an upper radiating element and a tank circuit which resonate at one frequency, and a helical element and associated sleeve member which resonate at a second frequency. While this apparatus is potentially shorter than a conventional sleeved dipole, it is still relatively large, and the usable operating bandwidth of the antenna about each resonant frequency is very small, such that this antenna system is not suitable for many potential dual-band applications such as cellular telephone.
U.S. Pat. No. 4,442,438 to Siwiak et al. discusses an antenna system comprising two quarter-wavelength helical antenna elements and a linear conductive member, which purportedly resonates at two different frequencies. However, the antenna disclosed in Siwiak et al. does not resonate at widely separated frequencies (the resonant frequencies disclosed were 827 MHz and 850 MHz), as the antenna is designed to broaden the antennas response to cover a single bandwidth of operation as opposed to providing for operation in two widely separated frequency bands.
Finally, additional helix antenna systems are disclosed in Japanese Patent No. 5-136623 and U.S. patent application Ser. No. 08/725,507, which discuss dual band operation through use of a conductive tube, and variable pitch windings, respectively. However, the mechanism for providing dual-band operation used in both these approaches, namely coupling between adjacent windings on the helix, typically results in a narrow operating bandwidth in the higher of the frequency bands and further may provide only limited design flexibility. Moreover, the antenna discussed in Japanese Patent No. 5-136623 also has a reduced effective aperture in the higher of the frequency bands.
Thus, in light of the above-mentioned demand for dual-band radiotelephones and the problems with current antenna systems for such radiotelephones, a need exists for radiotelephones with antenna systems that provide for good impedence matching in both the retracted and extended position for use in both bands of dual-band operation.
In view of the above limitations associated with existing retractable antenna systems for radiotelephones, it is an object of the present invention to provide retractable antenna systems with improved performance in both a high and a low frequency band.
Another object of the present invention is to provide retractable antenna systems which are conveniently small and which minimize the amount of radio frequency circuitry required while still providing for impedance matching in both a high and a low frequency band.
These and other objects of the present invention are provided by retractable antenna systems which comprise a helix antenna coupled to an extendible rod antenna. When the rod antenna is in the retracted position, the rod antenna is mostly, or completely, located within the housing of the radiotelephone, whereas when the radiotelephone operates with the rod antenna in the extended position, most, or all, of the rod antenna is pulled outside of the housing, thereby extending the helix antenna well away from the body of the radiotelephone. The antennas are configured to provide a xc2xc wavelength electrical length when retracted and a xc2xd wavelength electrical length when extended in the low frequency (such as AMPS) band. A matching circuit is provided using known techniques to provide a selected impedence in the extended position. The tap point where the rod antenna is connected to the helix antenna is at a position intermediate its ends, which position is selected to provide a desired impedance in the high frequency (such as PCS) band. As the tap position has a significantly greater effect on the electrical characteristics of the antenna in the high frequency band, the antenna of the present invention may be configured to exhibit approximately the same impedance in both the extended and retracted positions for both the high and low frequency bands, and, thus, the antenna systems of the present invention reduce or eliminate the need for additional impedance matching components.
In one embodiment of the present invention a radiotelephone having a retractable antenna system is provided including a housing and a transceiver disposed within the housing. A rod antenna is movably mounted within the housing and extendible from the housing so as to have an extended position and a retracted position. A helix antenna is provided having a first end and a second end. The helix antenna is connected to the rod antenna at a tap point intermediate the first end and the second end. A connection means electrically couples the transceiver to the rod antenna.
In a further embodiment of the present invention, the radiotelephone is a dual-band radiotelephone operating at a lower band and a higher band and the tap point is positioned to provide a selected impedance match at the higher band. The rod antenna and the helix antenna may provide a substantially xc2xd-wavelength radiating element at a center frequency of the lower band when the rod antenna is in the extended position. The rod antenna itself may be substantially a xc2xc wavelength radiating element at a center frequency of the lower band. Furthermore, the selected impedence may be substantially optimized for operation at the higher band.
In another embodiment of the present invention, the helix antenna is physically supported by the rod antenna. The helix antenna may be extended by slidably moving the rod antenna from the housing of the radiotelephone and the helix antenna is preferably substantially isolated from the housing of the radiotelephone when the rod antenna is in the extended position.
In a further aspect of the present invention, the connection means further provides means for electrically coupling the transceiver to the rod antenna and for causing the helix antenna to act as a dual band radiator when the rod antenna is in the retracted position. The connection means may include a parasitic element positioned adjacent the helix antenna. The helix antenna may also be wound in a non-uniform pitch.
In another embodiment of the present invention, a retractable dual-band antenna system for a radiotelephone having a housing and a transceiver is provided. The antenna system includes a dual-band antenna electrically coupled to the transceiver and configured so as to selectively radiate in both a higher and a lower frequency band. The dual-band antenna including a rod antenna movably mounted within the housing and extendible from the housing so as to have an extended position and a retracted position and a helix antenna having a first end and a second end and being electrically coupled to the rod antenna at a tap point intermediate the first end and the second end.
In a further aspect of the present invention a method is provided for designing a dual band retractable antenna for operation at a lower frequency band and an upper frequency band. A helix antenna having a first end and a second end is selected and a rod antenna is selected. An impedence matching circuit is designed for operations at the lower frequency band when the retractable antenna is in an extended position. Finally, a tap point is selected for electrically coupling the rod antenna to the helix antenna at a position intermediate the first end and the second end of the helix antenna.
Accordingly, the present invention provides dual-band radiotelephones and antenna systems providing improved performance for dual-band operations without the need for additional impedance matching circuits. The present invention provides this benefit based on the inventors"" discovery that an intermediate tap point electrically coupling the helix antenna and the rod antenna may be selected to optimize the impedence of the antenna system at the high frequency band without substantially affecting its impedence at the low frequency band.